1. Field of Invention
This invention relates to test fixtures for testing of printed circuit boards having test pins supported by multiple plates in parallel, spaced-apart array and, in particular, to fasteners and their use to keep multiple plates of the test fixtures at precise heights.
2. Brief Statement of the Prior Art
The automated testing of printed circuit boards to detect manufacturing flaws and defects is commonly performed with a mechanical press with one or two fixtures resting on the platens of the press. The fixtures have many spring pins which are located at preselected points for testing of a printed circuit board with pin-to-pin spacing being from 0.05 to 0.1 inch. The plates for the fixtures are usually predrilled with a grid-like distribution of holes at the aforementioned uniformed spacings.
Printed circuit board manufacturing capability, however, has advanced to very compact configuration, particularly with surface mounted components such as integrated circuits. These surface mounted components are mounted with connections which are at closer spacings than 0.05 inch, which is the closest pin-to-pin distance that can be practically provided with a uniform grid pattern of test pins. Accordingly, there is an increasing demand for fixtures which translate the geometrically uniform grid locations of spring test pins to off-grid locations.
A plurality of wire probes are supported in a fixture formed of two or more insulating plates separated by spacers, have been used as such fixtures.
In my prior patent, U.S. Pat. No. 5,307,560, issued May 3, 1994, I disclosed a method and apparatus useful in the automated loading of probes into a fixture. This method and apparatus utilizes a fixture formed of a plurality, usually three to four, rectangular plates which are retained by a plurality of standoffs or spacers between the plates. The fixtures require a close tolerance in the vertical separation of their uppermost and lowermost plates to insure a precise extension of the probes above the uppermost plate. This requirement is particularly important for the off-grid probes, which are canted in the fixture such that any imprecision in the vertical spacing can introduce a sufficient error in the lateral location of the end of the pins to fail to make contact with the test point on the printed circuit board.
In my prior patent, I disclosed that spacers can be placed between adjacent probe plates and assembly brackets can be placed over the edges of the probe plates and secured by screw fasteners. The vertical separation of the lowermost and uppermost plates are thus established by the sum of the thicknesses of the intermediate plates and the lengths of the 20 spacers.
The probe plates are formed of plastic sheet material, typically Plexiglass, which is available is standard thicknesses having a thickness tolerance of plus or minus 0.015 inch. When four plates are used to form the fixture, this tolerance can introduce an error in the height of the fixture as much as 0.06 inch. In an example of a probe which is canted at 15 degrees from vertical, the corresponding error in lateral location of the end of the probe can be from 0.015 to 0.025 inch, with the result that the probe can fail to make contact with the test point on the printed circuit board.